Seat support structures for vehicle seats must be lightweight and yet be sturdy to support the vehicle seat and its occupant during normal usage. Furthermore, seat support structures must also disperse extraordinary loads brought on by violent or abrupt maneuvering of the vehicle in the event of an accident or crash.
Aircraft seats must accommodate both a need for structural integrity and a need for minimum weight. Fuel consumption is partially a function of the weight of the aircraft. Therefore, aircraft seats are made as light as possible while still maintaining structural integrity. Generally, unitary seat frames are made of solid metal with carefully engineered voids and areas of reduced thickness to decrease weight while maintaining structural integrity. However, these seat members are resistant to bending and tend to shear or fracture on impact. This result is undesirable because broken members could injure passengers. Furthermore, broken seat members could puncture or otherwise degrade the integrity of fire-retardant barriers on the seats and impede evacuation of the aircraft.
Aviation safety standards generally require different g-load tolerances for an aircraft seat and the aircraft floor track to which the seat is mounted through a track fitting. For example, an aircraft seat must generally accommodate a 16 g-load whereas the tracks must generally accommodate a 9 g-load. Therefore, a crash generating greater than 9 g-loads will tend to separate the aircraft seat from its tracks. This is a highly undesirable result because dislodged seats could cause greater injury to the passengers. The letter "g" refers to one gravitational unit which equals 9.8 meters per second per second or 32.2 feet per second per second. Therefore, a 16 g-load equals sixteen gravitational forces.
Energy dissipating seat members dissipate energy as they are crushed in a controlled manner on impact. The dissipation of energy increases the passenger's likelihood of surviving an impact because a reduced impact force is transferred to the passenger. Further, the energy dissipation averts the tendency of the seat leg assemblies to shear.
Prior art discloses such energy dissipating devices. Generally, these devices consist of integral or non-integral formed devices that function on the basis of (1) compression stress activated devices, (2) tensile stress activated devices, or (3) a combination of compression and tensile stress dissipation devices.
U.S. Pat. No. 5,069,505 discloses a non-integrally formed, tensile stress activated energy dissipating system. The patent discloses a bent rear leg that distends when tensile stress is applied. As the rear leg distends, energy is dissipated as frictional heat energy through the body of the leg.
U.S. Pat. No. 4,861,103 discloses a non-integrally formed, compression activated energy dissipating device. The patent discloses a resisting structure for dissipating impact forces. The resisting structure consists of a shock absorber assembly in which a compression force is dissipated when a rod is forced into a block of synthetic plastic. The resisting structure links an aft leg member and a fore leg member. In the event of a crash, the rear and fore legs pivot forward.
U.S. Pat. No. 4,440,441 also discloses a non-integrally formed, compression activated energy dissipation device. The patent discloses a seat assembly with a tooled fore leg predisposed to collapse. When an impact occurs, the forward inertia of the seat causes the fore legs to collapse to the floor.
U.S. Pat. No. 5,224,755 and U.S. Pat. No. 5,282,665 disclose an integrally formed, compression stress activated energy dissipation device. Discussed is a seat leg assembly that utilizes an energy absorbing zone located between the fore seat support and the fore leg. The energy absorbing zone is positioned so that severe downward g-loading on the fore seat support will compress the fore seat support and the fore seat leg together, thereby dissipating energy through the energy absorbing zone.
With regard to the patents discussed above, integrated leg assemblies are favored due to the simplicity of manufacturing and maintenance. Multiple part assemblies require that the individual parts be manufactured and then be assembled into a leg assembly prior to attachment to the seat. This operation is labor intensive. Furthermore, non-integrally formed devices generally have pivot or attachment points which require maintenance and upkeep to ensure that the unit will function properly. Furthermore, non-integral units lack overall rigid strength to accommodate a plurality of varying forces that result from an air crash.
To operate as desired, compression activated devices generally require a unilateral force. However, aircraft crashes generally do not generate a single unilateral force but a plurality of random forces. A torsional force acting on a compression device or a shock absorber compression device would tend to skew the linear downward motion of the assembly, which would negate the controlled energy dissipation scheme of the device. Furthermore, upon activation, compression activated devices tend to cause the seat to encroach on the area defined by the immediately preceding seat. Although the devices may allow the leg assembly to remain secured to the aircraft's floor, the chair's occupant is potentially exposed to secondary injuries not caused by the initial impact.
Another tensile stress dissipating device utilizes a bent leg that distends when a tensile force is exerted on the seat. However, like the compression devices mentioned above, the bent leg devices also tend to encroach into the space of the immediately preceding seat. Furthermore, the device dissipates the impact force through the bent leg as frictional heat, further causing the leg to weaken and potentially fracture. Additionally, severe stress is conveyed to the floor track attachment of the leg device, potentially causing a failure of the floor connection resulting in loss of continuity with the floor track.